A phenomenon in wireless communication systems, such as digital radio or television transmission, is multipath propagation. This type of signal degradation occurs when a broadcast signal takes more than one path from the transmitting antenna to the receiving antenna so that the receiving antenna receives multiple signals. One of these multiple signals may come directly from the transmitting antenna, but several other signals are first reflected from buildings and other obstructions before reaching the receiving antenna, and are thus delayed slightly in phase from one another.
The reception of several versions of the same signal shifted in phase results in a composite signal actually being received at the receiving antenna. Two techniques may be used to deal with the multipath propagation of digitally modulated signals. These two techniques are inverse equalization and maximum likelihood sequence estimation (MLSE) detection.
In inverse equalization, an equalizer is implemented, digitally or otherwise, to reverse the propagation effects of multipath on the transmission waveform prior to detection. The equalizer is trained using blind equalization methods, decision feedback methods or by a transmitted training waveform.
There are two fundamental limitations of inverse equalization. The first is the equalizer length, which is a function of the multipath propagation impairment characteristics, namely echo delay and echo amplitude. Equalizer length is necessarily equal to or greater than, and often many times greater than, the multipath delay spread, depending on the amplitude of the multipath pre-echo and/or post-echo components. The second fundamental limitation of inverse equalization is that of 0 dB echo performance. In cases where the amplitudes of delayed signals are equal or nearly equal, the necessary equalizer is usually either unrealizable or impractical.
In MLSE detection systems, a fundamental limitation is complexity. In cases where the channel path count is large and the delay spread is much greater than the symbol interval, the list of survivors becomes unmanageably large, as does the length of the trellis required to represent each survivor. For example, several MLSE detection systems have been disclosed, such as the ones in Parr et al. (U.S. Pat. No. 5,263,026), Polydoros et al. (U.S. Pat. No. 5,432,821) and Parr et al. (U.S. Pat. No. 5,471,501).
In the Parr et al. '026 patent, a method for MLSE demodulation of a received serially modulated signal is disclosed, wherein multipath propagation impairment characteristics are estimated using a least mean square (LMS) algorithm. Rather than converging on an inverse of the multipath propagation impairment, the LMS algorithm converges on an estimate of the multipath propagation impairment. This channel estimate is integrally incorporated into the MLSE algorithm used to determine the symbols making up the serially modulated signal.
In the Polydoros et al. '821 patent, multipath propagation characteristics are incorporated into the survivor selection process used to accomplish data sequence selection. The survivor selection process is likewise based upon MLSE detection. Also in the Parr et al. '501 patent, MLSE detection is performed using an estimation of the multipath propagation impairment. As discussed above, the MLSE demodulation approach is limited by complexity.
A high definition digital television (HDTV) signal is also susceptible to multipath propagation impairment. The HDTV signal is a serially modulated signal based upon the standard set by the Advanced Television System Committee (ATSC) for terrestrial broadcast television in the United States. The ATSC digital television standard was determined by the Grand Alliance and was subsequently accepted by the broadcast community, the consumer electronics industry and the regulatory infrastructure.
The regulatory infrastructure has mandated a strictly scheduled transition of terrestrial broadcast television in the United States from the National Television System Committee (NTSC) or “analog” standard to the ATSC or “digital” standard. A significant investment is in place on behalf of the broadcast industry to support this planned transition. Similarly, many consumers have purchased ATSC television receiver equipment that include new ATSC system complaint DTV television sets and DTV television set-top converters.
However, the ATSC standard, in its present form, is deficient in its susceptibility to multipath propagation impairment. In side-by-side comparisons, ATSC reception, i.e., the new digital system, is often inferior to NTSC reception, i.e., the conventional analog system. Additionally, ATSC mobile reception suffers substantially more degradation due to multipath propagation impairment than NTSC mobile reception. Signal strength and signal-to-noise (SNR) ratios are typically not an issue, as unanticipated inferior reception manifests itself at high levels of received signal power and at high receiver SNR ratios. This fact, coupled with spectral analysis of received ATSC DTV signals, points directly to multipath propagation impairment as the cause of the inferior reception.
Various efforts have been made in the area of DTV reception. For example, Park et al. (U.S. Pat. No. 5,592,235) discloses combining reception, appropriate to terrestrial broadcast and to cable broadcast, both in a single receiver. Also included in these various efforts is Oshima (U.S. Pat. No. 5,802,241), which discloses a plurality of modulation components modulated by a plurality of signal components. Both of these references disclose the use of equalization. As discussed above, complexity of an equalizer is a fundamental limitation.
With respect to enabling the initial acquisition of digitally modulated signals that are severely distorted by multipath propagation impairment, decision-feedback equalizers (DFE) are not suitable. For this purpose, a reference or training waveform is typically introduced. The use of a reference sequence equalizer for equalizing GA-HDTV signals is disclosed in Lee (U.S. Pat. No. 5,886,748). Unfortunately, the Lee '748 patent does not overcome the limitations associated with inverse channel equalizers.